a) Field of Industrial Application
The present invention relates to a delay detection circuit and a low-noise oscillation circuit using such a delay detection circuit.
b) Description of the Prior Art
FIG. 4 shows a low-noise oscillation circuit constructed in accordance with the prior art. Such a low-noise oscillation circuit is also disclosed, for example, in SAKUTA et. al., "Improvement of Frequency Stability in Oscillator", Electronic Information Communication Society, Spring National Meeting, Vol. 1, page A-56, March, 1989.
In such a low-noise oscillation circuit, the oscillation output of a voltage controlled oscillator (VCO) 10 is supplied to a high frequency mixer 14, on one hand, directly, and on the other hand, through a delay unit 12. The high frequency mixer 14 multiplies the oscillation output of the VCO 10 by the output of the delay unit 12, the result being then supplied to a low-pass filter (LPF) 16. The LPF 16 removes high-frequency components associated with the multiplication from the output of the high frequency mixer 14. The LPF 16 then feeds the filtered voltage to the VCO 10 as a control voltage. The VCO 10 oscillates at a frequency corresponding to the control voltage.
If the oscillation output voltage of the VCO 10 is represented by EQU V(t)=A.multidot.cos (.omega.t+.phi.(t)),
the output voltage VP(t) of the LPF 16 may be represented by EQU VP(t)=1/2.multidot.A.sup.2 .multidot.cos.omega..tau.-1/2.multidot.A.sup.2 .multidot.sin.omega..tau..multidot.(.phi.(t)-.phi.(t-.tau.))
where A, .omega. and .phi.(t) are respectively the amplitude, angular frequency and variation of phase in the oscillation output voltage of the VCO 10 and .tau. is the delay time of the delay unit 12. As described in the above literature, the prior art reduces noise by setting the delay time T of the delay unit 12 to (2m-1).pi./2 (where m is an integer number) while at the same time using the output voltage VP(t) of the LPF 16 to control the oscillation frequency of the VCO 10.
However, the prior art has a disadvantage in that if the delay time .tau. of the delay unit 12 varies due to changes of ambient temperature or with age, the noise cannot be sufficiently reduced. In such an application where the oscillation frequency (angular frequency .omega.) of the VCO 10 is suitably changed, as in synthesizers, the delay time .tau. of the delay unit 12 may become offset from the optimum operating point, (2m-1) .pi./2, in association with the change of the oscillation frequency.
A technique of overcoming such a problem is disclosed, for example, in Japanese Patent Laid-Open No. Hei 3-140030. FIG. 5 shows a low-noise oscillation circuit as disclosed therein.
In such a low-noise oscillation circuit, the oscillation output voltage V(t) of the VCO 10 is applied to the high frequency mixer 14, on one hand directly, and on the other hand, through a voltage controlled delay unit 18. The output voltage of the high frequency mixer 14 is supplied to the VCO 10 as a control voltage. In this figure, high frequency components are ignored. In other words, the output of the high frequency mixer 14 is represented by VP(t). The voltage controlled delay unit 18 is one in which the delay time .tau. is controlled by voltage. The control voltage for the voltage controlled delay unit 18 is obtained by filtering the output VP(t) of the high frequency mixer 14 through the LPF 20. The cut-off frequency of the LPF 20 is set so as to remove the phase noise components, that is, the second right term of the aforementioned formula VP(t), from the oscillation output VP(t) of the high frequency mixer 14 so that only the DC components (first right term) can pass through the LPF 20. Since the DC voltage contained in the output voltage VP(t) of the high frequency mixer 14 depends on the delay time .tau. of the voltage controlled delay unit 18, the delay time .tau. of the voltage controlled delay unit can always be maintained at the optimum operating point, (2m-1).pi./2 if the sensitivity of the feedback loop containing the LPF 20 is sufficiently high.
The second prior art of FIG. 5 is superior to the first prior art of FIG. 4 in that the delay time .tau. can be always maintained at the optimum operating point (2m-1).pi./2. However, the second prior art requires a feedback loop for feeding the output of the high frequency mixer 14 back to the voltage controlled delay unit 18. Further, since sufficient sensitivity cannot normally be provided by only the LPF 20, the feedback loop requires a DC amplifier. Additionaly, although a voltage controlled electronic device such as varactor or the like can be used to provide a suitable voltage controlled delay unit, such a device is not suitable for use in providing integrated circuits. This becomes an obstacle to integrating and miniaturizing the system.